The word is originally Chinese so if you write it the way it’s usually pronounced you’d get “zuǒ” (Mandarin) but if you’re talking to a Cantonese speaker he won’t probably understand you so just say “zo2”. In either case, you won’t sound like a native speaker because English sound system if far from Mandarin or Cantonese. Still, many Chinese in America are Cantonese speakers so you should bear that in mind when you order the dish. Try to sound as close as possible to Cantonese. Saying “General So’s Chicken” is going to be just fine.